no problem
no problem
no problem...
i found the thing, and it was 5 gallon. the thing to remember i guess is that it depends on the size of your mantis...a tiny one would look slightly ridiculous in a 20 gallon (not to mention a waste of electricity and space), but the smaller the volume of water, the more "delicate" the water conditions have to be monitored, i'd think...
i do not believe a 5 gallon would be too small for a small mantis, either. i believe dr. caldwell over in his lab uses relatively small containers to house the many mantis he has.
good luck.